This invention relates to a multi-wavelength spectrophotometer, and more particularly to a multi-wavelength spectrophotometer in which the light entering a spectroscope chamber through an entrance slit is dispersed into various components at different wavelengths by dispersion means such as a diffraction grating or a prism and then introduced into a self-scanning detector to detect and measure the quantities of the respective wavelength components, whereby the measurement over a wide range of wavelengths can be made.
Although a spectrophotometer with a self-scanning detector (hereinafter referred to simply as detector) capable of performing the scanning over a wide range of wavelengths by use of only electrical means has several excellent advantages which the precedent spectrophotometer does not possess, it has encountered the problems that the dynamic range (hereinafter referred to as S/N ratio) of the detector is small, that an expensive detector having a long aperture area and a large number of elements must be used to cover a wide range of wavelengths, and that the influence of stray light is considerable in the measurement of shorter wavelengths and the stray light is difficult to eliminate. These problems make it difficult to put this kind of spectrometer into practical use.
These problems will be further detailed below. In the case where it is desired to design a spectrophotometer capable of covering ultraviolet to visible lights at wavelengths of 340 to 900 nm, a very important matter is the relationship between the characteristic of a light source and the spectroscopic characteristic of a detector. Usually, a tungsten lamp and a silicon photodiode are used respectively as a light source and a detector for the spectrophotometry over the above range of wavelengths. The energy spectrum of the tungsten lamp has its peak on the wavelength range of 900 to 1000 nm while the sensitivity of the silicon photodiode has its peak at the wavelength of about 800 nm. Accordingly, the composite spectroscopic characteristic of the light source and the detector has a remarkably large value, for the longer-wavelength region of 800 to 900 nm, resulting from a multiplied effect of the light source intensity and the detector sensitivity. For the shorter-wavelength region, on the other hand, the composite value is small and the quantity of stray light originating from the longer-wavelength region is considerable. Further, when the spectroscopic efficiency of a diffraction grating is taken into consideration, the detector output level for the longer-wavelength region in the ultraviolet to visible wavelength range would become 100 times as large as that for the shorter-wavelength region of the same range. Though the conventional detector which is not of self-scanning type can correct such an unbalance in output level over the wide range of wavelengths by changing the width of an exit slit and/or the gain of the detector in accordance with wavelengths to be measured, such a correction in the self-scanning type detector by similar operations is difficult since the scanning rate for one detector element is as high as several kHz to several MHz. In addition, the S/N ratio will be usually confined with a range of 200-1000 since the detector output for the incident light of high intensity is limited below a saturated output (usually several volts) while for the light of very low intensity a detector output smaller than the switching noise (several mV) of a MOS switch connected to each light receiving element in the detector cannot be discriminated. Therefore, the enjoyment of the maximum S/N ratio possible for a longer-wavelength region would result in that the S/N ratio for a shorter-wavelength region in vicinity of 340 nm is at highest about 2-10.
In addition, there is a further inconvenience that as described above, though the stray light originating from light of longer wavelengths considerably affects the measurement for the light of storter wavelengths whose quantity is small, it is very difficult to insert a stray light cut-off filter into the system. The reason is that the self-scanning detector has its dimension of several microns to several tens of microns and therefore the positioning of the filter requires a precision of almost the same order. Moreover, to cause a single detector to cover the above-mentioned wavelength range of 340 to 900 nm, the detector to be used must have the total length of about 66 mm when a concave diffraction grating with a radius of curvature R=200 mm and d=600 striae/mm is used, and the total length of about 33 mm even when a very small concave diffraction grating with R=100 mm and d=600 striae/mm is used. However, the detector presently available in the market has at best an interelement pitch of 20-50 microns and the total length of about 30 mm. The detector of this class is also very expense and therefore the range of wavelengths capable of being measured or covered must be narrowed to avoid too much expense.